1. Field of the Invention
The present invention relates to a temperature measurement system for monitoring the operating temperatures in a process. Particularly, the present invention is directed to a temperature measurement system wherein the temperature sensor is biased in the direction of thermal energy emitted by the body to be measured.
2. Description of Related Art
A variety of temperature measurement devices are known in the art for monitoring temperatures within a process. Of such devices, many are directed to measuring the temperature of metal being cast in a continuous casting process.
In a continuous casting machine (or “caster”), molten metal is poured into a cooled copper faced mold that controls the physical width and thickness of the finished product. Metal exits the mold in the form of a strand or slab having a thin shell of solidified metal with a core of molten metal. The strand continues into a secondary cooling zone to further solidify the metal. As the metal passes through the machine it is gradually cooled (secondary cooling) with water sprays or water/air mix sprays which are used to convert the molten metal from a liquid state to a semi-solid state as it changes direction from the vertical into the horizontal direction for handling and processing. The rate of cooling has a direct effect on the metallurgical characteristic of the metal being produced and there is an ideal cooling curve, known to those skilled in the art, which should be followed in order to achieve the best quality.
Unfortunately, because of the design of modern continuous casting machines, the best possible control of temperature is limited under the state of the art. From the exit of the mold to the horizontal point, the continuous casting machine length is divided into zones and preset water flow values are available to increase or decrease the volume of cooling water to those zones in order to achieve an exit temperature from the zone. Currently, the metal surface temperature is measured with optical pyrometers or similar devices. However, no successful attempts have been made to integrate that temperature to a predetermined curve such as an ideal curve, and imprecise cooling is the result. This is due in large part to the inability of traditional measurement systems to make accurate temperature readings in the casting environment, which includes temperature diverse flows of cooling fluids, gases, heated steam, other impurities, and metal in various stages of the liquid to solid transition.
It is desirable to keep the surface temperature of the metal controlled in a manner to prevent surface cracks or internal defects, which may occur if the metal is cooled too quickly, or prevent a breakout of molten metal from the core of the slab. Breakout is a major problem that occurs when the thin shell of the strand of material breaks, allowing the still-molten metal inside the strand to spill out and foul the casting system, requiring an expensive shutdown. Often, breakout is due to too high a withdrawal rate, resulting in the shell not having enough time to solidify to the required thickness. Alternatively, breakout can be due to the metal being too hot, which means that final solidification takes place below the straightening rolls and the strand breaks due to stresses applied during straightening. A typical breakout can cost a steel mill $250,000 and it is not uncommon to have two or three breakouts per month.
These failures result in costly further waste, processing, or expensive and dangerous consequences to personnel and equipment. In particular, for the steel industry, properly controlled surface temperatures result in better quality of steel and increased production rates.
To minimize breakouts, the conventional wisdom is to follow empirically established cooling processes that tend to overcool the slab as it passes through the caster. This is accomplished by controlling the flow of coolant with the assistance of a series of preset flow rates. The preset rates are adjusted to achieve an approximate temperature at various points along the caster. While slab temperature is sometimes checked with a measuring device, this device is not integrated into the coolant control system. It is common to only have a fixed pyrometer at the exit from the caster prior to the slab being cut. The resulting lack of accurate temperature control during formation of the shell can affect the product quality because of the inability of the system to follow a preferred cooling rate.
Attempts have been made in the art to address these deficiencies by providing a feedback mechanism to control the cooling of the slab as it passes through the caster. For example, U.S. Pat. No. 4,073,332 describes such a system. However, such systems suffer from certain deficiencies. A particular example of such a deficiency is the lack of temperature sensors that are suitable for the harsh environment inside of a caster, which tends to be extremely hot with very low visibility and high vibration. This deficiency is recognized in part by U.S. Pat. No. 4,073,332 at Col. 5, lines 6-10. Moreover, it has been recognized by others that the approach described in U.S. Pat. No. 4,073,332 is not practicable. For example, U.S. Pat. No. 4,699,202 recognizes the deficiencies of U.S. Pat. No. 4,073,332 at Col. 2, lines 8-21 in detail. The specifications of each of these patents are incorporated by reference herein in their entireties.
The need to improve the quality and the quantity of continuously cast materials with reduced down time is a driver in certain metal production industries, such as the steel industry. The state of the art still does not include a system for measuring the temperature of continuously cast metal with sufficient accuracy to allow for active control of the continuous casting process in a meaningful manner. There is still a long felt need in the art for such a system. There also remains a need in the art for such a temperature measurement system that is easy to make and use, and that is robust enough to reliably operate in harsh environments such as in a continuous casting process. The present invention provides a solution for these problems.